It is often necessary to scan documents having information on both sides of the document. Such a document is commonly referred to as a duplex document to distinguish it over simplex documents, documents with an image on only one side.
The prior art discloses various techniques for handling duplex documents. One method, used in electrophotography to copy duplex documents, is to sequentially expose both sides of the duplex sheet forming separate latent images of each side on a photosensitive surface. Each side of the document may be reproduced on a separate output sheet, or on both sides of a single output sheet. This technique, however, requires that the operator place one side of the duplex document on a platen, wait until the exposure is completed, and then turn the document over to expose the second side of the document. This process is slow and labor intensive.
Another method of scanning both sides of a duplex document passes the document through the same copy mechanism twice. This method, however, is slow since the documents must pass through the same copying mechanism twice.
Another method is described in U.S. Pat. No. 4,673,285 entitled "Optical Scanning of Duplex Documents." This method describes a duplex scanner with two optical paths, one to scan the front side of a document, and one to scan the rear side of a document. Both images are aligned and focused through a single movable lens. When scanning the front side of a document, the lens is aligned with the front side optical path and a charge couple device (CCD). The document's motion is then reversed and the lens is shifted into alignment with the rear side optical path and the same CCD. This method is slow since the same document is scanned twice.
A potential problem presented by a duplex scanner is to use one image processing system to support one image acquisition system. Therefore, in a duplex scanner, there would be two image processing systems to support two different image acquisition systems. This method is costly and may require several complex algorithms to be run on both the front and rear images.